Many kinds of materials manufactured by various molding processes have been used as skins for vehicle interior articles. For example, previously used materials include articles manufactured by vacuum-molding vinyl chlorides (PVC), articles manufactured by vacuum-molding or powder-molding thermoplastic olefins (TPO), articles manufactured by slush-molding thermoplastic polyurethanes (TPU), and articles manufactured by spraying or reaction injection molding (RIM) polyurethane raw materials (i.e., polyurethane liquid mixtures obtained by impinging and mixing two liquids, i.e. a liquid polyol and a liquid isocyanate, under a high pressure). In general, polyurethane raw materials are subjected to reaction injection molding to provide molded articles.
The molded articles of these materials have some defects, however. For example, PVC molded articles are undesirable since they release dioxin and chlorinated by-products while decomposing, thus polluting the environment. In addition, these materials are also hard to scrap or recycle.
TPO molded articles prepared by vacuum molding are undesirable as they feel hard and uncomfortable to touch. Molded articles prepared by powder molding are poor in production efficiency, as are the PVC molded articles, and thus these are inferior in cost-effectiveness since the molding processes require a relatively long time and consume a lot of energy.
TPU molded articles by slush molding are improved in terms of their feel and touch, but they are also poor in production efficiency and cost-effectiveness. The slush mold process also requires a relatively long time and a lot of energy.
Polyurethane molded articles which are manufactured by spraying polyurethane raw materials are an improvement over the conventional TPU molded articles produced by slush molding, because of the high productivity thereof. Since these polyurethane molded articles are generally manufactured from highly reactive raw materials, high productivity can be expected. Over-spray of the raw materials results in fumes and vapors which pose various industrial hygiene concerns. This is unavoidable, however, due to the process of spraying the raw materials. Consequently, the removal of the over-sprayed raw materials around the molds, which requires a considerable amount of time, is indispensable. As a result, the productivity is lowered due to the time spent for the removal of these over-sprayed raw materials. In addition, the quantities of raw materials which are used in the spraying process are increased because of the unavoidable over-spray of the raw materials. This also leads to an undesirable higher cost, as well as to pollution in the work environment.
The spraying process also has disadvantages in that the polyurethane raw materials which are sprayed are subject to the influences of atmospheric temperatures and humidity, and tend to absorb the water content from the air, or the air itself will form bubbles in the resultant articles. As a result of these influences, the raw materials tend to foam. In another situation, when molding a large vehicle interior article, it is necessary to spray the polyurethane raw material into the mold many times, which results in the layers of the polyurethane materials tending to have variable densities and hardness at the interfaces between each of the layers formed before and after the spraying of the polyurethane raw material. Thus, the surface of the resultant article feels inharmonious in touch. For these reasons, the resultant skin is varied in density, thickness, hardness and feeling.
A molded article obtained by the reaction injection molding (RIM) of polyurethane raw materials can have a highly designable appearance, because the unevenness of a surface can be precisely reproduced. In addition, the volume of the raw material in the mold is constant, so that the resultant article is stabilized in density, thickness and hardness. For these advantages, this method has been employed in a wide range of fields. In general, the thickness of a skin for use as the design surface of a vehicle interior article ranges from about 0.5 to about 3.0 mm, which is thinner than ordinary molded articles produced by a RIM process. Therefore, the raw material has higher flow resistance. In consideration of ensuring the filling of the raw material completely to the tip ends of the article, it is indispensable to lower the reactivity of the raw material. For this reason, the cure time (i.e. the time from the injection of the polyurethane raw material into the mold to the start of opening the mold) becomes longer, and results in poor productivity.
Skin-integrated vehicle interior articles such as instrument panels whose skins are used as design surfaces and door trims, and in particular, vehicle instrument panels, must not only have physical properties such as UV stability, wear resistance, expandability for air bag and durability, but also demand a high quality of external appearances and comfortable feeling (i.e. comfortable to the touch). Under these circumstances, it is desired to find a method for manufacturing an integrally molded laminate which comprises a skin as a design surface, a core material supporting the skin, and optionally, a semi-rigid polyurethane foam for reducing the impact between the skin and the core material, in which the method is characterized by a shorter cycle time and the yield is improved, giving higher productivity and at lower production cost.
Materials which feel comfortable in terms of touch and which can be manufactured in high productivity and at lower production cost have been demanded as skins for the interior articles of vehicles, in particular, automobiles. Previously, there have been several proposals known for these materials and their processes of manufacturing them.
The reference JP-A-52-142797 discloses a method for manufacturing a polyurethane elastic molded article, which comprises using a specified polyol and a specified aromatic diamine. In this reference, a method of molding a polyurethane elastomer in a shorter time is described which requires a specified polyol and a specified aromatic diamine. According to this publication, the molded articles are relatively large exterior parts, with weights of 3 to 10 kg or more, such as bumpers for vehicles, etc. In the Examples of this publication, articles with a thickness of 4 mm, having a Shore A hardness of as relatively high as 84 or more, are produced. By comparison, the skins in accordance with the present invention for use as the design surfaces of vehicle interior articles are not of this type.
JP-A-53-86763 discloses a method for forming a foam layer in tight contact with a skin. This method comprises the steps of forming a skin by the reaction injection molding of a polyurethane raw material in the cavity of a mold; allowing the skin to remain in the mold and removing the first core part; placing a second core part which is capable of shaping a cavity equivalent to the thickness of a foam layer, instead of the first core part; and injecting a foaming material into the cavity, thereby forming the foam layer in tight contact with the skin. This reference proposes a method of molding an interior article integral with a skin. However, this publication only describes that the material for the skin is of the urethane type, and does not disclose any details about the composition of a urethane material suitable for this method, the density of urethane and/or the surface hardness of the skin.
The reference JP-A-2003-19056 discloses a seat cushion using a seamless skin material and a method for manufacturing the same. This publication proposes a seat cushion comprising a foam article having practically the same contour as that of the seat cushion, and a seamless skin material adhered and integrated into the surface and side faces of the foam article. However, this reference also only describes that the material for the skin is of the urethane type, and does not describe or disclose any details about the composition of a urethane material suitable for this method, the density of urethane and/or the surface hardness of the skin material.                Patent Literature 1: JP-A-52-142797        Patent Literature 2: JP-A-53-86763        Patent Literature 3: JP-A-2003-19056        
At this time, there is no satisfactory skin which feels comfortable in touch, and has a high quality of outer appearance for application as a skin for a vehicle interior article, and which can be manufactured in high productivity and at lower cost without having any adverse influence on the work environment and/or nature. Also, there is no satisfactory method currently available for manufacturing a skin-integrated laminate comprising such a skin.